Hussar
Legend
Does that count for NPCs too? In 4e monsters don't get this benefit (and it is smaller, only negative bloodied, but that is still a lot, PCs are rarely killed outright).
By the rules, yes, this applies to NPC's. Although, I think that most DM's, like myself, ignore that rule and simply rule that anything that goes to 0 dies. However, by the rules, no, everything is supposed to get death saves.
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Umm, if you killed someone with a pillow or a knife [MENTION=23751]Maxperson[/MENTION], you'd be charged with manslaughter, not assault, and, certainly if you attempted to smother someone with a pillow, they certainly could charge you with assault with intent.
But, arguing from a position of legalities, isn't really going to get anywhere.
You're trying to apply a specific definition of "lethal" as in the legal definition, to the argument, and I think that's where we're getting tripped up, because, to me, lethal means, "something that kills you". A lethal dose of a drug is enough of a drug to kill you. If it didn't kill you, then it wasn't a lethal dose.
Being shot with a gun may be lethal, if you die. If you didn't die, then it wasn't a lethal wound.
But, all that being said, you're still ignoring the fact that you cannot narrate any wound until after everything is done. Say you fireball the target and drop it below 0 HP. What wounds has it actually taken? You cannot narrate it as a lethal wound, because it hasn't died yet. In fact, until that 3rd death save is failed, you cannot narrate any wound as being life threatening. Because, otherwise, it doesn't make much sense that your arterial spray is healed 100% by the next day.

Now, lots of DM's ignore this and fair enough, but, by the mechanics of the game, you cannot actually narrate any wound until combat is completely finished or the target is truly dead. Because, frankly, you cannot know what any of those wounds are until either of those two conditions are met. Either they are minor wounds that are easily ignored (healed in 24 hours) or they are mortal wounds.
Y'know, that might be a better word to use - mortal rather than lethal. A fireball can, potentially, inflict mortal wounds, but, by all odds, unless it outright kills that target, it's extremely unlikely. All "knocking out the target" really does is save you a Medicine check to stabilize a target. Which, odds are, will survive on their own anyway.
In my current Dragon Heist game, because Waterdeep has very serious penalties for killing, regardless of reason, I've ruled that NPC's die automatically to anything other than melee attacks. But, that's my house rule. It's certainly not how 5e is presented.
Complaining about retconning the fiction in 5e is a pretty big hole to dig. THere's a TON of retroactive mechanics in 5e.